With the development of digitally-controlled metamaterial technologies, intelligent reflecting surface (IRS) has emerged as a cost-efficient technique to achieve smart and reconfigurable radio environment for future wireless communication systems. Specifically, IRS is a programmable metasurface consisting of a large number of passive reflecting elements, whose amplitude and/or phase shifts can be individually controlled in real time, thereby enabling dynamic control over the wireless propagation channel for a variety of purposes (e.g., passive beamforming, multi-path diversity, interference nulling/cancellation, and so on) [1]}, [2]}, [3]}. Moreover, IRS dispenses with transmit and receive radio frequency (RF) chains and reflects ambient signals only passively, which requires low hardware cost and energy consumption. Thus, IRS has been extensively investigated in assorted wireless communication systems, such as orthogonal frequency division multiplexing (OFDM)-based wideband communications [4]}, [5]}, multi-antenna and/or multi-user communications [6]}, [7]}, multi-cell networks [8]}, [9]}, non-orthogonal multiple access (NOMA) systems [10]}, [11]}, relaying communications [12]}, [13]}, physical-layer security [14]}, [15]}, unmanned aerial vehicle (UAV) communications [16]}, [17]}, mobile edge computing (MEC) [18]}, [19]}, etc.
